2. Transport through the root Flashcards
Why is water needed in plants
- Mineral ions & sugars are transported in aqueous solution
- Water is a raw material of photosynthesis
- Cooling effect (by transpiration)
- Turgor pressure - hydrostatic skeleton
Simple overview of water movement through plants
- Water uptake near root tips (HIGH WATER POT)
- Water enters xylem
- Water moves up xylem
- Water moves from xylem to leaf cells
- Evaporation of water into leaf air spaces
- Transpiration of water vapour through open stomata into air (LOW WATER POT)
(see s7)
What is water potential
The tendency for water molecules to move within & between cells
Water moves from region of HIGHER water pot to region of LOWER water pot
plasmolyzed. flaccid. turgid
What has the highest water potential
Pure water
How is water potential of a solution LOWERED
Presence of solutes lowers the water pot of a solution
What does having water potential of 0 mean
Maximum water potential
- Therefore all solutions have a water pot less than 0 (-ve number)
Uptake of water process
A passive process
By osmosis
Uptake of minerals
Can be passive or active
By diffusion or active transport (respectively)
How does water potential inside the cell affect movement of water
- If water pot inside the cell is low (higher salt concent) water will move in by osmosis
- If water pot inside the cell is high (low salt concent) water will move out by osmosis
What happens in low external water potential
Water moves out of the cell
- Plants can survive this for short periods, as they can shrink the cell membrane away from the cell water
- The cell is said to be plasmolysed
s14 for dia of movement of water from root hair to xylem
Adaptations of root hair cells
- Very thin surface layer (wall & plasma membrane) - so diffusion & osmosis can happen quickly
- Microscopic in size - meaning they can penetrate easily between soil particles
- Large SA:V ratio
- Concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water & the cell
see s18… for structure of a root dia
How does water travel through the root (via the root cortex) into the xylem
Water travels through the roots (via the root cortex) into the xylem by 2 different paths:
- The symplast pathway
- The Apoplast pathway
- Vacuolar pathway
Symplast pathway
Water moves through the living parts of the cell - the cytoplasm.
- The cytoplasms of neighbouring cells connect through plasmodesmata. Water enters cells through the plasmodesmata (small channels in the cell walls).
- Each cell further away from the roots has a lower water potential so water is drawn through the plant.
- Water moves through the symplast pathway via osmosis
Apoplast pathway
Water moves through the non-living parts of the cell - the cell walls & intracellular spaces
- The walls are very absorbent & water can simply diffuse through them, as well as pass through the spaces between them.
- Water moves via diffusion as its not crossing a partially permeable membrane (simply cell to cell or through intracellular spaces)
- The water can carry solutes & move from areas of high hydrostatic pressure to areas of low hydrostatic pressure. This is an eg of mass flow
- This is the fastest movement of water
Vacuolar pathway
The same as the symplast pathway when the water moves through the cells vacuoles in addition to the cytoplasm
- This is the slowest route for water
What is the reason for the presence of the Casparian strip
Not fully understood but scientists think that:
- this may help the plant control which mineral ions reach the xylem
- it plays a part in increasing root pressure
What happens to the Casparian strip as the plant ages
Thickens (as more suberin is deposited) except in cells called the passage cells, allowing for further control of the mineral ions
see s32 for dia of apoplast, symplast, vacuolar pathways
What is the Casparian strip
An impermeable layer of suberin - a waxy material
As a result, all water in apoplast pathway is forced into symplast pathway
Where is the Casparian strip
The endodermis cells in the root contains the Casparian strip
Explain the casparian strip (+following events)
- When water in the apoplast pathway reaches the endodermis cells in the root, its path is blocked by a waxy strip in the cell walls - the Casparian strip
- It is a waterproof, impermeable layer. This is bc of the waxy layer of suberin in walls of endodermal cells
- In order to cross the endodermis, water that has been moving through cell walls in apoplast pathway, must now move through the cell surface membrane & into the cytoplasm - forced into symplast pathway
- This is useful bc it means the water must go through a cell membrane, which are partially permeable & so are able to control whether or not substances in the water get through
- As a result, the cell membrane can remove any toxic solutes from the soil, only allowing necessary water molecules/mineral ions to enter
- Once past this barrier, the water moves into the xylem
Evidence for active transport in root pressure
- Effect of Cyanide
- Effect of Temperature
- Reactant availability
- Guttation
Evidence for active transport in root pressure: Effect of Cyanide
Cyanide stops the mitochondria from working, therefore root pressure decreases
Evidence for active transport in root pressure: Effect of Temperature
Root pressure increases as temp increases & decreases as temp decreases, suggesting an enzyme controlled chemical reaction
Evidence for active transport in root pressure: Reactant availability
If oxygen levels or respiratory substrate levels drops, root pressure decreases
Evidence for active transport in root pressure: Guttation
Sap & water will move out of cut stems, suggesting they are actively pumped out & not drawn up by transpiration
How does water enter a plant
- Water must get from the soil, through the root & into the xylem to be transported around the plant
- Water moves through root hair cells & then passes through the root cortex, including the endodermis, to reach the xylem
- Water is drawn into the roots via osmosis - meaning it travels down a water potential gradient
How does water potential affect movement of water into a plant
- Water always moves from areas of higher water potential to areas of lower water potential - it goes down a water potential gradient
- The soil around roots generally has a high water potential (theres lots of water there) & leaves have a lower water potential (bc water constantly evaporates from them)
- This creates a water potential gradient that keeps water moving through the plant in the right direction, from roots (high) to leaves (low)
Which of the 2 is the most efficient pathway
Both are used, but the main one is the apoplast pathway bc it provides the least resistance
Complete movement of water
Water moves through the root into the xylem, then up the xylem & out of the leaves
What happens after the water moves through the root into the xylem
- Xylem vessels transport the water all around the plant
- At the leaves, water leaves the xylem & moves into the cells mainly by the apoplast pathway
- Water evaporates from the cell walls into the spaces between cells in the leaf
- When the stomata (tiny pores in the surface of the leaf) open, the water diffuses out of the lead (down the water potential gradient) into the surrounding air
- The loss of water from a plant’s surface is called transpiration
What is the movement of water from roots to leaves called
Transpiration stream
What are the 3 mechanisms that move the water in the Transpiration stream
- Cohesion
- Tension
- Adhesion
3 mechanisms that move water in Transpiration stream: Cohesion & tension
Cohesion & tension help water move up plants, from roots to leaves, against the force of gravity
1. Water evaporates from the leaves at the ‘top’ of the xylem (transpiration)
2. This creates a tension (suction), which pulls more water into the leaf
3. Water molecules are cohesive (they stick tg) so when some are pulled into the leaf, others follow. This means the whole column of water in the xylem, from the leaves down to the roots, moves upwards
4. Water enters the stem through the root cortex cells
3 mechanisms that move water in Transpiration stream: Adhesion
Adhesion is also partly responsible for the movement of water.
1. As well as being attracted to eachother, water molecules are attracted to the walls of the xylem vessels
2. This helps water to rise up through the xylem vessels
